1. Field of the Invention
This invention relates to optical systems, and more specifically, to an ellipsometric optical measurement system incorporating a resonator to enhance the resolution and sensitivity of the system.
2. Description of the Related Art
Resonator-enhanced optical inspection systems, storage devices and other optical systems, such as those described by U.S. Pat. Nos. 6,653,649, 6,700,840, 6,714,295, 6,717,707, 6,778,307 issued to Applicant Clark and others, the specifications of which are incorporated herein by reference, provide improved resolution, surface detection and other performance improvements in traditional optical systems and provide new types of optical systems that were not available prior to the inventions disclosed therein.
In particular, the combination of a partially reflective surface with a surface under measurement to form a Fabry-Perot resonator provides increased resolution, improved differentiation of surface height and differentiation of surface characteristics, and improved feature detection over the prior available techniques.
However, the reflection measurement systems disclosed in the above-referenced U.S. patents are generally directed toward surface measurements that center on the performance of the surface of interest and other reflectors as ideal reflectors. As such, significant surface feature characteristics, such as transmission vs. absorption and polarization changing effects are not detected by such systems, or their effect is essentially combined with other effects, so that the additional surface characteristics are not uniquely detected. When transmission or absorption characteristics are measured, such as in above-incorporated U.S. Pat. No. 6,653,649, they are combined in the total reflected amplitude or phase at the Fabry-Perot external detection surface and are limited to determining specific characteristics, such as the thickness of a refracting layer.
Ellipsometry is a well-known technique for measuring the complex reflection characteristics of a surface, which provide information about material and structure of a surface. Ellipsometry is also used for thin-film layer investigation, where multiple thin film layers yield complex reflective changes. By controlling the polarization of light directed at a surface at a non-normal incidence angle and then variably filtering the reflected light for a particular polarization angle or interfering the reflected polarizations, the polarizing effect of the surface at the particular angle of incidence can be mapped by finding the minimum and maximum amplitude points as the receive filter is rotated. The mapping is performed over several angles of incidence on the surface, and the complex refractive index components (n—index of refraction) and (k—extinction coefficient) can be determined from the “ellipsometric parameters” which is the phase angle and the ratio of the refractive indices at the surface boundary.
The ellipsometric detection can be performed by rotating a polarizer to determine the major axis and minor axis of an amplitude ellipse that describes the polarization behavior, or by interfering two orthogonal polarization states of the reflected beam, as in a Beam Profile Ellipsometer (BPE) such as that described in U.S. Pat. No. 5,181,080 to Fanton, et al. The measurements are typically performed over several incidence angles in order to determine the complex surface reflectivity.
However, standard ellipsometers are limited by the resolution of the system, the phase sensitivity of an interferometric measurement and/or the accuracy of the amplitude-based “manual” ellipse technique described above.
It would therefore be desirable to improve the performance of an ellipsometer by increasing its sensitivity and resolution. It would also be desirable to add polarization measurement capability to optical systems disclosed in the above-referenced patents, as well as other resonator-enhanced optical systems, in order to provide measurement of complex optical surface properties or detection of surface features that generate changes in such optical properties. It would finally be desirable to detect ellipsometric parameters of a surface in an efficient and rapid manner.